1. Field of the Invention
The present invention relates to a method of manufacturing a II-VI Group compound semiconductor device and a III-V Group compound semiconductor device used as a light-emitting device, for example, a UV-emitting laser diode, blue light-emitting laser diode, UV-emitting diode, or blue light-emitting diode and more specifically, to a method of manufacturing a low-resistance p-type compound semiconductor from a III-V Group compound semiconductor and a II-VI Group compound semiconductor from by doping p-type compounds thereinto as impurities.
2. Description of the Related Art
Studies on blue light-emitting elements have been generally conducted using ZnSe, which is a II-VI Group compound, SiC, a IV-IV Group compound, or GaN, a III-V Group compound.
Of the types of compounds mentioned above, it was recently found that a gallium nitride series compound [Ga.sub.x Al.sub.1-x N (where 0.ltoreq.x.ltoreq.1)] semiconductor exhibits excellent semiconductor light emission at room temperature, and therefore much attention is now being paid to the GaN series semiconductor.
A blue light-emitting basically has a structure in which n-type, and i-type or p-type GaN series semiconductors each represented by general formula Ga.sub.x Al.sub.1-x N (where 0.ltoreq.x.ltoreq.1) are stacked in turn on a sapphire substrate.
There are several well-known methods for growing a III-V Group compound, such as the metalorganic chemical vapor deposition (MOCVD) method, the molecular beam epitaxy method, and the hydride vapor phase epitaxy method. As an example, the MOCVD method will be briefly described. In this method, a metalorganic compound gas serving as a reaction gas (for example, trimethyl gallium (TMG), trimethyl aluminum (TMA), or ammonium) is introduced into a reaction container (vessel) in which a sapphire substrate is placed. Then, while maintaining the epitaxial growth temperature as high as 900.degree. C.-1100.degree. C., an epitaxial film of a III-V Group compound is grown on the substrate. By supplying suitable impurity gas during the growth of the film according to circumstances, a multilayer made of the n-type and p-type III-V Group compound semiconductors can be manufactured. In general, Si is a well-known n-type impurity; however in the case of a GaN series compound semiconductor, there is a tendency for the semiconductor to exhibit the n-type characteristics even without doping an n-type impurity. Some of the well-known examples of p-type impurities are Mg and Zn.
There can be proposed a method described below, as an improved version of the MOCVD method. When a III-V Group compound semiconductor is directly epitaxial-grown on a sapphire substrate at a high temperature, the surface condition of the crystals, and the crystallinity will be extremely degraded. In order to avoid this, before the compound is grown at the high temperature, an AlN buffer layer is formed on the substrate at a temperature as low as about 600.degree. C., and then the compound is grown on the buffer layer at a high temperature. The fact that the crystallinity of GaN can be remarkably improved by the above-mentioned technique is disclosed in Published Unexamined Japanese Patent Application No. 2-229476. Meanwhile, the authors of the present invention disclosed in Japanese Patent Application No. 3-89840, prior to the present application, that a gallium nitride compound semiconductor having a better crystallinity can be formed when a GaN buffer layer is used than when a conventional AlN buffer layer is used.
However, a blue light-emitting device employing a blue-color-emitting element including a GaN series compound semiconductor has not yet been developed as a practical device. This is because p-type III-V Group compound semiconductor having a sufficiently-low-resistance cannot be produced by any of the conventional techniques, and therefore a light-emitting element having various types of structure such as p-type double hetero, single hetero, etc. cannot be manufactured. In the case where an epitaxial film is formed by the conventional chemical vapor deposition method, even if the film is grown while doping p-type impurities, it is impossible to make III-V Group compound semiconductor characteristic p-type. And also a semi-insulation material having a high resistivity of 10.sup.8 .OMEGA..multidot.cm or higher, i.e., an i-type semiconductor may be obtained. Consequently, at present, the blue-light-emitting element having a structure of the p-n Junction diode cannot be achieved, but a so-called MIS structure is the only one known structure for the blue-color-emitting element, in which structure, a buffer layer, an n-type film, and an i-type film are formed on a substrate in the mentioned order.
Published Unexamined Japanese Patent Application No. 2-257679 discloses a method for reducing the resistance of a high-resistance i-type semiconductor as little as possible to convert into a type close to a p-type one. In this method, a high-resistance i-type GaN compound semiconductor layer into which Mg was doped as a p-type impurity is formed on the top of the multilayer of the GaN compound semiconductor. Then, while maintaining the temperature of the compound not higher than 600.degree. C., electron beams having an acceleration voltage of 5 kV-15 kV are irradiated on the surface so as to reduce the resistance of the layers located in the surface portion within a depth of about 0.5 .mu.m. However, with this method, reduction of the resistance can be achieved only up to the point where electron beams can reach i.e. a very thin surface portion. Further, in the method, the electron beams cannot be irradiated on the entire wafer while scanning the beams, and consequently the resistance cannot be uniformly reduced in the desired surface. Further, this method entails the problem of a very low reproducibility, i.e., the resistance value changes every time electron beam is irradiated to the same sample. With this method, it is impossible to constantly produce blue-light-emitting elements having a high efficiency.
Study is being directed not only to III-V Group compounds, but also to II-VI Group compounds in order that they can be put into practical use. As in the case of the GaN compound production method, the chemical vapor deposition method such as the MOCVD can be used to form a II-VI Group compound semiconductor.
Growth of ZnSe by the MOCVD method will be briefly described. In this method, an metalorganic compound gas (diethylzinc (DEZ), hydrogen selenide (H.sub.2 Se), etc.) is introduced as reaction gas into a reaction vessel in which a GaAs substrate is placed. Then, while maintaining the epitaxial growth temperature at about 350.degree. C., ZnSe is grown on the substrate. During the growth, an appropriate impurity gas is supplied to the vessel to form an n-type or p-type ZnSe semiconductor. Examples of the type of substrate are GaAs and ZnSe. Further, Cl is a well-known n-type impurity, and N is also the well-known p-type impurity.
However, as in the case of the before-mentioned p-type GaN compound, a sufficiently low-resistance p-type ZnSe compound cannot be produced by this conventional technique, and therefore a light-emitting element having various types of structure such as double hetero, single hetero, etc. cannot be manufactured. In the case where eptaxial-growing is performed by the conventional chemical vapor deposition method while doping p-type impurities, the obtained ZnSe compound semiconductor will be a compound having a high resistivity of 10.sup.8 .OMEGA..multidot.cm or higher.